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Abstract: This study presents a transformerless topology for a grid-tied single-phase inverter capable of performing the simultaneous maximum power point tracking of two independent and series connected photovoltaic sources. This topology is derived from the neutral point clamped multilevel inverter in half-bridge configuration. The use of a half-bridge topology reduces the leakage current to very low values, whereas the multilevel topology presents an output voltage quality similar to that of a full-bridge inverter. To simultaneously track the maximum power of both photovoltaic sources, a generation control circuit is used. With this topology, it is possible to improve the performance of the converter under partial shadowing conditions, very common in photovoltaic facilities operating in residential areas. A 5 kW prototype of this topology has been implemented and tested in the laboratory.
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1 Introduction
Photovoltaic (PV) and renewable energy sources (RES) have experimented a great development in recent years [1], mainly because of the growing concern about climate change and the oil price increase, which has led many countries to adopt new regulations to promote this kind of energy.
In power converters for RES, and especially in grid-connected PV inverters, efficiency and cost are key factors [2]. Many of these inverters use an isolation transformer between the PV panels and the grid, but these low-frequency transformers are bulky and expensive. Furthermore, they produce additional losses [3, 4]. Using a two-stage topology with high-frequency isolation can reduce the size and price of the transformer, but the overall efficiency of the system is reduced, since at least two cascaded power stages are used (dc-dc + inverter). Therefore a large number of transformerless inverter topologies have been proposed in the past years [5]. These inverters are cheaper, more compact and more efficient [4, 6] than their counterparts.
Regarding the size of grid-connected PV inverters, a change of paradigm has been observed in the past few years. Large central inverters (above 100 kW) are being substituted by string inverters around a few kilowatts that process the energy of a small group of PV panels, improving the maximum power point tracking (MPPT) of a PV system, as the modules could be exposed to different solar irradiation levels. In this context, the use of single-phase inverters up...